explanation in comments
<?php
$posts = get_field('related_post');
// this is getting an image from the current post
$related_image = get_field( 'product_image' );
if( $posts ): ?>
<?php foreach( $posts as $post): // variable must be called $post (IMPORTANT) ?>
<?php setup_postdata($post); ?>
<div class="large-4 columns">
<a href="<?php the_permalink() ?>">
<?php
/*
$related image here is the image you got above
Not an image associated with the post used in this nested loop
if you want to get an image from this related post you need to get it here
*/
?>
<img src="<?php echo $related_image['sizes']['related-image-size']; ?>">
</a>
</div>
<?php endforeach; ?>
<?php wp_reset_postdata(); // IMPORTANT - reset the $post object so the rest of the page works correctly ?>
<?php endif; ?>
hope that helps
